Dr. John and Dr. Jane Doe?
Drs. John and Jane Doe?
Many variations are possible, and some more common in social than business contexts.
For example, a wedding invitation could be addressed to "The Doctors Doe" or "Drs. John and Jane Doe".
In traditional formal usage (in the U.K., at least), the wife of the gentleman John Doe is always styled Mrs. John Doe. Her given name and other titles are not used.
So your letter should be addressed to Dr. and Mrs. John Doe.
Debrett’s is considered something of an authority on such matters.